The present invention pertains to drill bits, more specifically to drag-type drill bits and to the cutter devices or cutters which are mounted on the bodies of such bits. The bits may be of the full bore or corehead type.
A typical bit includes an integral bit body, typically comprised of one or more body members of either tungsten carbide matrix material or a suitable metal, relatively non-frangible as compared to tungsten carbide, e.g. steel. A plurality of cutter devices is mounted on the bit body. Each such cutter device typically has a stud portion, which is mounted in a pocket or recess in a bit body member and defines one end of the device, and a cutting formation generally adjacent the other end.
The cutting formation is located on what may be considered a leading side of the device, so that as the bit is rotated in its intended direction in use, this cutting formation engages and drills the earth formation. The opposite side of the device is considered the trailing side, and the device may further be considered to have a pair of lateral sides, generally opposite each other, and interconnecting the leading and trailing sides. The leading side may also be referred to as the forward side, and the trailing side referred to as the rear side.
In use, tremendous forces are exerted on the cutting devices in the forward-to-rear direction. In some cases, cutting devices have been broken by these forces.
Accordingly, it is desirable to maximize the strength of these devices. However, this goal must be balanced and coordinated with other objectives and/or limitations.
For example, the design of the bit body necessarily places limits on the maximum transverse dimensions of the stud portion of the device, in both the forward-to-rear and lateral directions. Furthermore, the devices are often arranged in rows extending generally radially across the working end face of the bit body, and the performance of the bit is affected by the number of devices which may be placed in a row, or in other words, the spacing of the cutting formations of the devices in a given row. In general, at least for some types of earth formations, it is desirable to place these cutting formations as close together as possible, i.e. to place as many devices as possible in a given row on the bit body. However, a limiting factor on this objective may be that there be adequate thickness of bit body material left between each two adjacent cutter devices.
Another important consideration is that it should be possible to manufacture a number of such cutter devices to fairly accurate dimensions, i.e. within fairly close tolerances, but without undue expense in the manufacturing process.
In the past, the stud portions of typical cutter devices have been generally cylindrical, except for a small groove or keyway on the rear or trailing side, which may cooperate with a small protrusion in the respective pocket of the bit body to properly index or orient the cutter device with respect to the bit body.
Such cylindrical stud portions have not satisfied the above-mentioned goals or objectives to the extent desirable. If the diameter of such a cylindrical stud portion were chosen to correspond to the maximum forward-to-rear dimension considered practical for a given bit design, it would be necessary either to leave less than optimum amounts of bit body material between the pockets for adjacent cutter devices, or to space such adjacent pockets and cutter devices apart by an amount greater than that which would permit the desired number of cutter devices per row on the bit body. On the other hand, if a smaller diameter were chosen, so that more cutter devices could be used in a row without unduly thinning or weakening the portions of the bit body between adjacent cutter devices, the devices may not be strong enough in the forward-to-rear direction, and may break off in use, as described above.
Another potential problem with such conventional cylindrical stud portions is that the aforementioned keyway could represent a weakening concavity in the cross section.
Yet a further problem with conventional cutter mounting bodies having cylindrical stud portions relates to the form of the cutting formation and its manner of mounting on the mounting body. The cutting formation per se may comprise a relatively thin layer of superhard material. Although polycrystalline diamond is by far the most common such material, others might be feasible. For purposes of this specification, "superhard" will refer to materials significantly harder than silicon carbide, which has a Knoop hardness of 2470. For convenience, this thin layer of superhard material is usually first applied to one side of a circular disc, e.g. of sintered tungsten carbide, which serves as a carrier for the thin layer of polycrystalline diamond or the like. Then, the opposite side of this carrier disc is bonded to the mounting body.
As previously mentioned, the mounting body includes the aforementioned stud portion adjacent one end, and when this stud portion is installed in its respective pocket in the bit body, the other end of the mounting body will protrude outwardly from the bit body. It is to the leading side of the mounting body adjacent this other end that the carrier disc must be bonded. Thus, it is not feasible for the mounting site on the mounting body to be cylindrical because of the difficulty of forming the adjacent side of the carrier disc with a matching concave cylindrical surface.
Accordingly, it has become conventional to begin with a cylindrical workpiece for the mounting body and then grind a planar surface on the leading side of the cylindrical member to which a flat-sided carrier can readily be bonded. This planar mounting surface or flat is disposed at an angle with respect to the centerline of the cylindrical workpiece, which also builds in a back rake angle to the cutting face.
This leads to several problems. First, in order for the mounting flat to be wide enough to receive a carrier disc of a desired diameter, the cylindrical workpiece on which such flat is machined, and which defines the stud diameter, must have an even greater diameter. Furthermore, the mounting flat will necessarily intersect the cylindrical portion of the mounting body in an elliptical line, which has proven to be extremely troublesome in actual practice for several reasons.